The purpose of hunger is to stimulate food intake to ensure adequate nutrients for metabolism.
The components of hunger can be simplified and broken up into two categories - mechanical and chemical.
The mechanical elements are related to the stomach, primarily the stretch receptors that detect expansion of the stomach as food comes in.
Remember it takes a little while for the food to get from your mouth to the stomach, which is why you can consume huge amounts of food that then lead to discomfort before the receptors have a chance to send their message back to the brain.
As food is broken down and digested, the stretch dissipates and the receptors begin to switch off until all the food is gone.
The chemical process has to do with the release of macronutrients into the bloodstream.
As foods are digested and broken down into these nutrients, they pass into the bloodstream to be transported around the body.
This increase in nutrients in the bloodstream signals that food has been digested and is available for use.
Just after a meal when the mechanical system is saying it’s full, the chemical system might still be signalling a low nutrient state, as the new food hasn’t been digested and absorbed.
This explains why you might still feel the need to eat more after a meal, particularly if you’ve eaten quickly, only for that feeling to dissipate.
Likewise, a couple of hours after a large meal the mechanical receptors might be quiet as the majority of food has passed, yet the chemical receptors note large amounts of nutrients in the bloodstream (with some more to come).
This explains why you can comfortably eat food after a meal, even when you aren’t hungry.
The mechanical and chemical systems interact with hormones that tell the brain whether you need to source more food or that you are satisfied.